Opiate-like peptides were isolated from beef adrenal medulla. These peptides show met-enkephalin-like immunoreactivity, but have a higher molecular weight than met-enkephalin suggesting the existence of a immunoreactive precursor. Metenkephalin and its analogs, Tyr-D-Ala-Gly-Phe-Met-Nltz and FX 33-824 CH decreased the mean arterial blood pressure (MAP) in rats, and this effect could be blocked by naltrexone. In contrast, the reduction of the MAP elicited by Beta-lipotropin 60-65, Beta-endorphine and gly-gly-phe-met was not curtailed by naltrexone. The met-enkephalin-like peptide isolated from beef adrenal medulla also decreases MAP. The natural peptide is considerably more potent than the above analogs of met-enkephalin.